Jm. Didymus et al., INTERACTION OF POLY(ALPHA,BETA-ASPARTATE) WITH OCTADECYLAMINE MONOLAYERS - ADSORPTION BEHAVIOR AND EFFECTS ON CACO3 CRYSTALLIZATION, Langmuir, 11(8), 1995, pp. 3130-3136
Langmuir monolayers have been used to study the interfacial adsorption
of poly(alpha,beta-aspartic acid) peptides. At pH 5-6, isotherms for
the positively-charged octadecylamine (CH3(CH2)(16)NH3+) monolayer sho
wed expanded Limiting areas per molecule (30-35 Angstrom(2)) compared
to octadecylamine spread on pure water (approximate to 20 Angstrom(2))
, suggesting strong adsorption and partial intercalation of a 4.1 kDa
peptide between surfactant molecules. Branched-chain peptides of diffe
rent molecular masses (2 and 5 kDa) and a linear 7.1 kDa molecule had
similar effects, but the alpha-aspartate monomer had no influence on t
he isotherms. Thus, polycarboxylate molecules were required for effect
ive adsorption although the number of acidic groups on the polymers wa
s not important. The presence of calcium ions in the subphase, at pH 5
-6, appeared to have little effect for the branched peptides studied,
but the linear 7.1 kDa poly(alpha,beta-aspartate) showed enhanced, if
variable, intercalation (30-75 Angstrom(2)). This was probably a resul
t of conformational changes rendering the molecule more predisposed fo
r adsorption. No effects were observed with negativelycharged or neutr
al monolayers, indicating that the driving force for adsorption at the
air/water interface was electrostatic. The presence of poly(alpha,bet
a-aspartate) in supersaturated calcium bicarbonate solutions resulted
in vaterite nucleation rather than calcite crystallization as in contr
ol experiments. Calcium carbonate crystallization under compressed oct
adecylamine monolayers produced oriented vaterite crystals, whereas in
the presence of the peptides, nonoriented vaterite aggregates were fo
rmed at the monolayer/solution interface.